My wife and I went through the D-visa process last year. Yes, purchasing an apartment or a house fully satisfies the accommodation requirement for the retirement visa. You just need to present the title deed (notary act) when you apply at your home country's Bulgarian embassy. However, before you sign anything, read up on
Buying Property in Bulgaria as a Foreigner: Step-by-Step Guide and Common Pitfalls because foreign ownership laws regarding land can be tricky depending on whether you are buying a house with a yard or just a standard apartment.